The present invention relates to a method and system for diagnosing deterioration of an exhaust emission control catalyst, which is provided inside an exhaust passage of an engine, and particularly to a method and system for diagnosing deterioration of an oxidative catalyst, which includes an HC adsorbing part.
For the purpose of purifying NOx (nitrogen oxide), HC (hydrocarbon), and CO (carbon monoxide) within exhaust gas to be discharged from engines (e.g., diesel engines and gasoline engines), exhaust emission control catalysts (e.g., three-way catalysts, oxidative catalysts, and NOx storage and reduction catalysts) are provided inside exhaust passages of the engines. When such exhaust emission control catalyst deteriorate, NOx, HC, and CO are discharged outside a vehicle in which the engine concerned is installed, without being purified. Therefore, it becomes necessary to detect the deterioration of the exhaust emission control catalyst. Among such exhaust emission control catalysts, for exhaust emission control catalysts including an oxidative catalyst part for purifying HC by oxidation, since an amount of oxidative reaction heat produced when HC is oxidized becomes small due to the progression of the deterioration, the deterioration of the oxidative catalyst part can be determined by detecting the decrease in oxidative reaction heat. For example, JP2010-112220A discloses a deterioration diagnosing system of an exhaust emission control catalyst including such an oxidation catalyst part.
With the method disclosed in JP2010-112220A, an exhaust heat rate obtained by multiplying an exhaust gas temperature by an exhaust gas flow rate is calculated for parts of the exhaust passage on an entrance side and an exit side of the exhaust emission control catalyst, individually. Then an oxidative reaction heat rate produced within the exhaust emission control catalyst is calculated based on a difference between the exhaust heat rates on the entrance side and the exit side. When an integrated value of the oxidative reaction heat rates in a predetermined period of time is smaller than a threshold determined for the deterioration diagnosis, the exhaust emission control catalyst is determined as deteriorated.
In other words, in JP2010-112220A, regarding the deterioration diagnosis based on the oxidative reaction heat produced within the exhaust emission control catalyst, the oxidative reaction heat rate calculated based on the exhaust gas flow rate is used as a diagnostic parameter, so as to reduce a false deterioration determination of the exhaust emission control catalyst which occurs when an oxidized amount changes due to a change of the exhaust gas flow rate and the detected oxidative reaction heat varies.
Meanwhile, due to the tightening of exhaust gas regulations in recent years, discussion has occurred regarding introducing exhaust emission control catalysts provided with an HC adsorbing part having a function to adsorb HC discharged from the engine when a temperature of the HC adsorbing part is low, and discharging the adsorbed HC when the temperature of the HC adsorbing part is high. Such an exhaust emission control catalyst provided with the HC adsorbing part can temporarily adsorb HC when the exhaust emission control catalyst is not activated and HC cannot sufficiently be purified, and it can discharge and purify the adsorbed HC after the exhaust emission control catalyst is activated. Therefore, HC discharged outside the vehicle can be reduced.
In performing the deterioration diagnosis of the exhaust emission control catalyst based on the oxidative reaction heat rate, the diagnosis is preferably performed when the oxidative reaction heat rate is obtained many times. With the method disclosed in JP2010-112220A, under a warmed-up state of the engine (paragraph[0037]), “the deterioration diagnosis is permitted when one of the following conditions is met: a period of time for which the heat rates are integrated is over a predetermined period of time; a travel distance of the vehicle in a period of time for which the heat rates are integrated is over a predetermined value; and the integrated value of the output of the vehicle in a period of time for which the heat rates are integrated is over a predetermined value.” In other words, the deterioration diagnosis is performed when an absolute value of the heat rate becomes large.
On the other hand, since temperature sensors generally cause detection errors, if the heat rate is calculated by integrating the oxidative reaction heat rates per unit time over a comparatively long period of time, the error of the temperature sensor also accumulates and the diagnostic accuracy degrades.